A lamp of the type to which the present invention relates, and over which the present invention is an improvement, is described in U.S. Pat. No. 3,700,881 Slomski. In the arrangement of this U.S. patent, the lamp bulb is cemented into the end of the neck of the reflector which surrounds the entire end of the lamp. A free or hollow space arises in this construction in advance of the cementing position between the end of the lamp bulb and the neck of the reflector. This hollows space forms a heat retaining zone, the purpose of which is to provide for a low temperature gradient with respect to the length of the space and thus prevent thermal stresses. The heat storage capability is improved by forming the neck of the reflector in conical shape, so that the space becomes smaller or tapers towards the outer end of the lamp. A single cap is used to close off the arrangement, secured by a cement, and surrounding the neck of the reflector. The lamp current leads are conducted outwardly through an opening in the side wall of the cap. Such a lamp is useful as a direct current arc discharge lamp in which the anode is surrounded by the neck of the reflector and of the heat retention space. The current supply of the end of the lamp in the direction in which light is emitted is returned towards the end cap within the reflector. In spite of this rather complex construction, the lifetime of the arrangement is between 25 to 50 hours, which is short for commercial utilization of this type of lamp of projection purposes.